Head-mounted display devices (also called herein head-mounted displays) are gaining popularity as means for providing visual information to a user. Some of the head-mounted display devices are provided with eye tracking sensors for updating visual information based on the information from the eye tracking sensors.
However, in conventional head-mounted display devices, eye tracking sensors are located away from an optical axis of display devices due to the limited space available in conventional head-mounted display devices. In particular, even more restricted space is available for head-mounted display devices configured for augmented reality or mixed reality operations. Such configurations have limited the accuracy in determining gaze directions of eyes of a wearer due to off-axis illumination and sensing in eye tracking.